The present invention relates to a bumper beam for motor vehicles.
Motor manufacturers are concerned with improving passenger safety and with reducing the cost of repairing vehicles in the event of impacts, while at the same time avoiding any significant increase in the weight of these vehicles.
For safety particularly in the event of a frontal impact, manufacturers are optimizing the various parts of the vehicle so as to guarantee maximum absorption of kinetic energy by mechanical deformation of zones away from the cabin and so as to strengthen the cabin in order to protect the passengers.
For repairing vehicle parts damaged after a frontal impact, manufacturers are looking first of all to raise the speed at which irreversible damage occurs to as high as possible a speed, this speed being of the order of 5 km/h, and then to limit the damage to the ends of the vehicle, that is to say to zones which can easily be repaired, up to speeds of the order of 15 km/h.
The safety objective is generally achieved by providing the vehicle with longerons, the gradual deformation of the ends of which absorbs the kinetic energy of the vehicle, the zones of the longerons located closest to the cabin and the cabin itself being designed to deform very little.
However, given the limited space in motor vehicles, manufacturers, are having to develop structures which guarantee a good level of energy absorption from the end of the vehicle up to the non-deformable zone while at the same time avoiding dead zones, that is to say zones which cannot deform axially and absorb significant deformation work.
Furthermore, manufacturers have reduced the costs of repair in low-speed impacts by modifying the bumpers or the scuff moldings of the vehicles, particularly by strengthening the bumper beam and inserting, between this beam and the longerons, deformable mechanical elements known as energy absorbers which are designed to crumple under a force which is appreciably lower than the force required to deform the longerons.
In the event of a low-speed impact, all that is therefore required is for a limited number of parts to be replaced, thus limiting repair costs.
Repairs are also improved by strengthening the bumper beam so as to raise the speed of impact without damage and to guarantee good distribution of forces in the event of a higher-speed impact.
What happens is that the bumper has to transmit the forces to the energy absorbers and then to the longerons of the vehicle in the later phases of the impact.
The way in which this structure, which consists of the bumper beam and of the energy absorbers, works has also to be extremely stable with respect to angles of impact more or less steeply inclined with respect to the longitudinal axis of the vehicle.
In an attempt to satisfy these criteria, numerous solutions have been implemented to date, but they lead to an appreciable increase in the weight of the vehicle and in cost.
One of the known solutions consists in fitting steel blades in bumpers made of synthetic materials and in incorporating energy absorbers between the bumper and the longerons of the vehicle. However, the energy absorbers used to date are not always satisfactory and entail adding additional parts, which increases the weight of the vehicle.
To avoid this drawback, energy absorbers are known which are made of aluminum, which allows the weight to be reduced, but their energy-absorption properties are generally not easy to control and often insufficient, while the material costs and cost of manufacture of such absorbers remain high.
Also known are energy absorbers made of steel with a relatively complex geometry which fulfills the safety criteria and allow a slight reduction in weight. However their cost of manufacture also remains very high.
An object of the invention is to avoid these drawbacks by proposing a bumper beam for motor vehicles which, at the same time, makes it possible to improve the energy absorption and also to reduce both the cost of repair in the case of a low-speed impact, and the weight of the vehicle.
The subject of the invention is therefore a bumper beam for motor vehicles, characterized in that it comprises a front sole plate and a rear sole plate which are vertical and made of at least one metallic material, at least one core made of metallic material and positioned between the two sole plates and, at each end of said core, an energy absorber formed of a hollow body made of metallic material running at right angles to the sole plate and connected to at least said front sole plate or to said core, the metallic materials of this core and of the hollow body of the energy absorbers having a ratio between the elastic limit and the breaking stress lower than that of the metallic material of said sole plates and lower than 0.9.
According to other preferred characteristics of the invention:
the metallic material of the sole plates is a steel with a very high elastic limit higher than 400 MPa or an aluminum with a very high elastic limit of above 250 MPa,
the sole plates are made of the same metallic material and have different thicknesses,
the metallic material of the front sole plate has a ratio between the elastic limit and the breaking stress lower than that of the metallic material of the rear sole plate,
the bumper beam has two parallel cores stretching between said sole plates,
the thicknesses of the sole plates and of the cores are different and the thickness of the sole plates is preferably greater than that of the cores;
said core comprises a succession of alternating projecting parts and recessed parts running at right angles to the longitudinal axis of this core,
the distance between the top of the projecting parts and the bottom of the recessed parts of said core is between {fraction (1/20)}th and one half of the period of the projecting parts or recessed parts,
the distance between the bottom of the recessed parts of the two cores is between 0 and half the period of said projecting parts or recessed parts,
the hollow body of each energy absorber has a cross section in the shape of a four-armed cross, the arms extending in twos in the continuation of one another and making an angle of 90xc2x0 between them.